This invention relates to load pull testing of power transistors and noise testing of low noise transistors using automatic impedance tuners used in order to synthesize reflection factors (or complex impedances) at the input and output of said transistors.
A popular method for testing and characterizing RF components (transistors) for high power or low noise operation is “load pull” and “source pull”. Load pull or source pull are computerized measurement techniques employing impedance tuners and other test equipment, such as signal sources, test fixtures to house the DUT (device under test, transistor) and input and output power meters (FIG. 1). To measure noise figure and noise parameters of a low noise device a similar setup is used (FIG. 2), in which the signal source is replaced by a calibrated standard noise source and the power meter by a sensitive noise figure analyzer, following a signal amplifying, low-noise amplifier. The tuners in particular are used in both setups, in order to manipulate the electrical impedance conditions under which the DUT is tested.
A popular family of electro-mechanical tuners, the “slide-screw tuners”, use adjustable metallic probes (slugs) inserted into the transmission media of the tuners [1]. Said transmission media in RF and microwave frequencies is a slotted straight section of coaxial airline, typically manufactured as a parallel-plate structure (FIG. 3). Starting at the center of the Smith chart (FIG. 4, (4)) when the RF probe (1) is inserted further (6) into the slot of the slabline (3) the capacitive coupling with the center conductor (2) increases and so does the reflection factor (7). When the RF probe is moved along the slot and parallel to the axis of the slabline (8) the phase of the reflection factor changes until a desired reflection factor (9) is reached. This way the whole area of the Smith chart (5) can be covered (FIG. 4).